Breaking the Photoabsorption‐Relaxation Trade‐Off: Flat‐Band Engineering for Efficient Photothermal Conversion Cocrystals

ABSTRACT Along with the pursuit of efficient solar energy utilization, organic cocrystals have emerged as potential photothermal media due to their high light–heat conversion efficiency and flexible designability. However, the trade‐off between strong absorption and rapid nonradiative decay in cocrystals remains a longstanding challenge. Here, we investigate the electronic structures and photoexcited‐state dynamics of charge‐transfer cocrystals (coronene‐F x TCNQ; x = 0, 2, 4) and establish a design strategy that combines a flat‐band with an indirect‐gap to overcome this limitation. This synergistic electronic configuration enables broadband absorption up to 2500 nm and directs photoexcitation into ultrafast nonradiative channels, yielding peak photothermal conversion of 73% under 808 nm excitation. Ultrafast spectroscopy, combined with theoretical analysis, reveals enhanced joint densities of states and modified transition selection rules underpin phonon‐assisted conversion. Demonstration applications incorporating them in solar desalination, sewage purification, and photo‐thermoelectric conversion achieved a maximum solar utilization efficiency of 96.6%, demonstrating high performance and operational robustness. This work provides a rational design guideline for designing next‐generation cocrystal photothermal materials.